1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method. More particularly, the present invention relates to an image processing apparatus configured to reduce color unevenness caused by individual variations in the ejection characteristics of a plurality of nozzles that eject ink.
2. Description of the Related Art
In print heads used in inkjet printing apparatus, individual variations are sometimes exhibited in the ejection characteristics (i.e., the ejection volume and ejection direction, for example) of a plurality of nozzles, due to manufacturing errors and other factors. When such variation exists, printed images become more susceptible to density unevenness.
In the related art, one established process for decreasing such density unevenness involves using head shading technology such as that disclosed in Japanese Patent Laid-Open No. H10-13674 (1998). Head shading is a technology that corrects image data according to information regarding the ejection characteristics of individual nozzles. By means of such correction, the number of ink dots that are ultimately printed is increased or decreased for each nozzle, and the density in the printed image can be made nearly uniform across the nozzles.
However, when head shading technology as described above is used, if two or more types of ink are combined for color reproduction, then a phenomenon will occur wherein the coloration in the areas printed with the nozzles having non-standard ejection volumes will differ from the colors that should be printed. In other words, the above might cause inaccurate color, herein referred to as color difference.
By way of example, consider the case of printing a blue image using nozzles exhibiting standard ejection volumes for cyan ink, and greater-than-standard ejection volumes for magenta ink. In this case, the magenta ink with the greater-than-standard ejection volumes will form dots on the print medium that are larger than the cyan dots. If such print heads are corrected by means of head shading (i.e., an HS process), then magenta will be printed using fewer dots than the standard number of dots. In other words, the number of magenta dots will be less than the number of cyan dots. As a result, the blue image regions will contain a mixture of solid cyan dots of standard size, as well as overlapping dots wherein cyan dots are printed inside larger magenta dots. The coloration in such regions will be different from the coloration in a blue image printed using cyan dots and magenta dots of standard size and number. This occurs because the ratio of the print medium occupied by solid cyan, the ratio occupied by solid magenta, and the ratio occupied by blue resulting from overlapping cyan and magenta, all differ between the above two images. Such variation in the surface area ratios occupied by respective colors occurs not only because of variations in ejection volume variation, but also because of variations in ejection direction. In other words, even if density unevenness in solid cyan images or solid magenta images is resolved by the head shading of the related art, the variation in ejection characteristics will still lead to color difference in blue images expressed by overlapping combinations of these colors. Furthermore, since the degree of color difference differs among the regions printed by nozzles with different ejection characteristics, different color are perceived in individual regions that should have the same coloring, which is noticed as color unevenness.